Ionic amplifier



s. RUBEN 2 IONIC AMPLIFIER Jan. 29, 1935.

Filed Dec. 16, 1.9515

FIG.1. SM/2a kw J a b 4 a 7 C 6'2 f 3 6 2b i: I: 10 L L Li: I I I i l I i i i I v III II I I! III I I I 'y/ I II I 4 I I l/ A INVENTOR SAMUEL RUBEN ATTORNEY Patented Jan. 29, 1935 UNITED STATES PATENT OFFICE romc AMPLIFIER Samuel Ruben, New Rochelle, N. Y. Application December 16, 1933, Serial No. 102,667

18 Claims.

Thisinvention relates to an ionic discharge amplifier.

This application may be said to be a continuation in part of my applications bearing Serial Numbers 634,994, filed September 27, 1932; 641,334 filed November 5, 1932, and 669,503 filed May 5, 1933, relating to grid controlled ionic-amplifiers. An object of the invention is the provision of an ionic discharge device capable of being used as an amplifier and specifically to provide a device capable of being used as a power detector in a radio receiving circuit without the requirement of an auxiliary audio amplifier.

Another object is to provide a means of ob-- taining a sensitive or graduated control of an ionic discharge throughout the entire current output range.

A specific object is to obtain such a control electrostatically by the use of an initially negatively biased grid element and with minimum grid current fiow.

A further object is the provision of an amplifier device in which the higher power output obtainable by the use of an ionized metal vapor such as mercury or mercury amalgam can be controlled by an electron discharge within the amplifier device.

An object, is to provide an electrical discharge tube allowing a load impedance and a power output and sensitivity greater than that obtainable from vacuum type amplifier tubes of comparable structure.

A specific object is to provide an electrical dischargetube having an atmosphere of restricted vapor pressure and in which the mean free path length is greater than the distance between the cathode and control element.

An object of this invention is to provide in an electron discharge tube a limited vapor atmos phere insufficient to allow conduction within certain areas by ionic conduction, but sufiicient to allow a reduced electron space charge.

'Still another object is to employ in tubes of the type described an atmosphere which is the resultant product of two materials one having a relatively low vapor pressure, for instance, cadmium, and one having a relatively high vapor pressure such as mercury, the ratio of composition being such as to prevent an excessive rise of pressure which might tend to cause an arc-like discharge under operating conditions. Such arclike discharges are caused by ionic bombardment in devices where the pressure is such that the meanfree path length is less than the distance between the elements maintained at a potential difference.

- Other objects will be apparent from the disclosure and from the drawing in which,

Fig. 1 shows a tube of the invention in an operable circuit, and

Fig. 2 is a top view, showing the arrangement of the elements.

The tube may also be used in other circuits and for such other uses as combined oscillator and detector, audio and radio frequency amplifier, 5 voltage amplifier in connection with such devices as photo cells, relays, etc.

The tube physically comprises an electron emission cathode, an anode, an ionizable medium, a control grid surrounding the cathode, a shield 10 or space charge grid surrounding the control gr'id, Q a third grid surrounding the shield grid and facilities at the terminals of the electron emiss'ion areas for confining the discharge directly to the space between the cathode and anode through 5 the grids. The spacings between the tube elements are of fundamental. importance; are dependent upon such factors as vapor pressure and mean free path length and controLthe operation and effectiveness of the tube.

In general, the device operates with a direct electrostatic control of an electron discharge with means of shielding the control electrode from an ionic discharge resulting vfrom impact of the con-'- trolled electron discharge with the vapor atmos- 5 phere.

In thermionic discharge tubes of the prior art employing a'thermionic cathode in a space charge reducing atmosphere such as mercury, only a trigger type of control is obtained. Once the cur- 30 rent fiow starts, the grid has little influence on the discharge, the change in grid voltage merely varying the thickness of the sheath of positive ions surrounding the grid. Where the grid supply potential variations are of a practical order, 35 the plate potential must be disrupted to extinguish the discharge. The trigger potential is determined by the initial amplification factor of the tube. This trigger characteristic has prevented the use of tubes of this type in circuits such 40 as shown in the'drawing or-in other audio amplifier circuits requiring a graduated or non-distorted amplification of an applied control potential to the grid control element.

Furthermore, in such grid controlled tubes, only 45 a very low plate potential such as a maximum of fifteen volts is applied to the plate, whereas in the tube of my invention, much higher voltages, such as in the order of two hundred volts or more, may be applied to the plate.

In the present tube a complete graduated and sensitive control equivalent to that had in a pure electron type of discharge is obtained. In this tube the electron discharge surrounding the oathode is directly controlled by a grid located close to the cathode, preferably within the "cathode fall space", at a distance less than the mean free path length of the electrons in the ionizable medium. The control grid is surrounded by a positively charged grid which serves a two-fold purpose: It tends to lower the impedance and space charge and reduces the migration of positive ions to the control grid.

Generaly speaking, it may be said that up to this point the present tube is somewhat similar to that described in my co-pending application bearing Serial Number 669,503.

However, the tube of this invention differs from the tubes described in my co-pending applications. Instead of having a fairly definite limit at which the control grid may be placed from the cathode, as represented by the cathode fall space described in the prior applications, the area of controllable electron discharge adjacent the cathode has been extended. This allows a more practical structure and has the effect of reducing sputtering of the elements.

This extension of the electron discharge or elctrostatically controllable area has been accomplished by the use of a material having a vapor pressure much lower than the mercury utilized in my prior tubes and by employing two grids surrounding the control grid. A factor of importance is that the thrid grid tends to further reduce ionic current fiow to the control grid. Such ionic fiow would tend to render the control grid ineiiicient (electrostatically) It may be said that the tube of this invention is preferably a detector and that the tubes of my two preceding applications referred to are primarily amplifiers.

As in the tubes of my co-pending applications above referred to, the surrounding anode may be located at a distance beyond the cathode fall space or at a distance greater than the mean free path length of the electrons in the ionizable medium; also as in my other tubes, the discharge is confined directly to the space between the cathode and anode, through the grids.

When looking downward in the tube, (when operating) it is observed that the area between the cathode and control grid is dark; that the area between the control grid and the second grid (first positively charged grid) is dark with negligible visible discharge; that there may be a faint luminosity in the area between the second and third grids, but that the area between the third or outer grid and the anode, is highly luminous, due to the increased ionization of the vapor in this space. Thus, the control grid isoperated in a relatively ion free atmosphere, as can be further noted from low grid current flow when biased negatively, and the smooth and stepless control of the plate current output. If the potential on the positively charged second grid should be raised beyond a critical value, probably represented by the sum of the ionization potential of the gas and the field voltage drop across the preceding grid or grids, intense or localized ionization can occur in the space between the cathode and the grids, with change from a dark to a luminous area. Substantial ionization in the space surrounding the control grid makes its control ineffective.

The relative luminous effects noted in this tube are not due to means used in the prior art, where, for example, a self supporting discharge was used with a control grid located in the cathode dark space, or where screens have been used to separate the discharge 'into two definite classes. The negligible amount of ionization in my tube near the cathode is due to the use of an electrode distance less than the electron mean free path length which with the spacial relations used is obtained by use of the cadmium amalgam as a source of ionizing medium.

From the foregoing, it is to be noted that certain fundamental factors are essential in relation to the tube and its circuit. These may be stated as:

l. The potential applied to the second grld should be less than is necessary to intensely ionize the space between it and the cathode and is determined by the combined effect of ionization potential of the gas, the gas pressure and the field voltage gradient across the first grid.

2. That the potential applied to the second grid should not be of such a magnitude as to force a localized ionic discharge from it to the cathode through the control grid.

3. A third grid, to assist in reducing electron impedance and further shield the control grid from the ionic sheath, is necessary- 4. That the source of an ionizable medium or vapor should not have a large pressure rise with temperature and should be such that the mean free path length of the electrons in the ionizable medium under all operating conditions is greater than the distance between the control electrode and the cathode or the space between the grids. The ionizable medium should have limited or restricted maximum pressure rise.

The three grids should preferably be as close as practicable. It is desirable that they should have approximately the same number of turns.

In order to reduce sputtering of the grids. due to ionic impact, I have found it desirable to coat the grids with a material having a low atomic number, such as beryllium, carbon or' aluminum. One of the most practical methods is to coat the grids with deflocculated graphite to which has been added about twenty per cent aluminum powder. The latter prevents scaling or peeling of the graphite coating, probably due to its leaflike structure and expansion factor.

In order to prevent the tube from becoming inoperative or running away due to temperature rise and resultant increase in vapor pressure as when mercury is used, and to allow a mean free path length greater than the space between the control electrodes, Ihave found it very desirable to use an amalgam of mercury and cadmium in place of the straight mercury. As the cathode fall space varies with the internal atmosphere pressure (see my co-pending application bearing Serial Number 669,503) as does also the free mean path length, unless a means is provided to prevent the large rise in vapor pressure of the mercury in the tube, due to the heating caused by the cathode and plate energy dissipation, or unless an external cooling means is provided, the tube will become inoperative with an arc-like discharge between cathode and anode and no control will be had. By use of a cadmium-mercury amalgam, a'smaller bulb can be used and the starting time cut to a fraction of what it would be with pure mercury. In addition, with a proper proportion of cadmium, the pressure does not rise to excess value under any operating condition. The cadmium-mercury amalgam being in a hard solid form, is chemically stabile and easy to handle.

Ihave used other mercury amalgams such as the alkaline amalgams sodium and potassium.

conditions and are relatively unstable as compared with the cadmium amalgam.

The inert gases, neon, helium, argon, etc., can

V plete mixture.

be used where long tube life is not a primary requisite.

As the exact spacing of the grids to the cathode and especially of the control grid to the cathode is dependent upon the vapor pressure under maximum operating conditions, it will be seen that the choice of quite important. Naturally, vary with the size of the bulb, power in the tube and location of the tube with respect to associated apparatus, 11 as in relation to the kind of ionizing medium used.

In the typical tube described in my co-pending application bearing Serial Number 669,503, in which mercury vapor derived from pure mercury is used, and witha 2.5 watt cathode heater loss, the cathode fall space was such that the control grid was spaced at a distance approximately only .010" from the cathode. Whereas, in the tube of this invention, using a bulb of identical capacity, with an ionizing medium composed of approximately e' hty per cent cadmium and twenty per cent mercury, the control electrode can be spaced .040" from the cathode with negligible ionization in the space therebetween. The space between the control grid and second grid and between second and outer grid is also of this magnitude.

This allows a less critical tube to be constructed and permits the use of a smaller bulb with decreased lag in the time required for the tube to come to equilibrium. The tube is also less sensitive to temperature variations.

Anotheradvantage derived from the use of a cadmium-mercury amalgam in the tube, is that the plate voltage supply can be connected to the tube prior to the heating of the cathode without the destructive cathode disintegration due to localized arcing which is common in mercury vapor tubes.

charge in the tubes but insufficient to allow the formation of localized arc discharge.

As to the proportions of mercury to cadmium, I have found that in a bulb having a capacity'of '72 c. c. and a power dissipation or loss (combined cathode and anode) of 12 watts, that a. compound or mixture composed of 20.4% mercury and 19.6% cadmium affords a satisfactory atmosphere source. matter from these figures to determine the proportion of mercury to cadmium for a given size bulb and power loss to give an equivalent pressure.

There is an indication that the cadmium and mercury form a definite compound such as cadmium mercuride HgCd1, for in volatilizing from pill container, the deposit on the inner wall of the bulb indicates the volatilization of the com- This would also account in part for the limited or restricted vapor pressure rise with temperature.

An essential difierence between this cadmiummercury amalgam tube and mercury vapor tubes of the prior art, is that the vapor pressure is just sufficient at normal operating conditions, to reduce the electron space charge, but insufiicient to allow uncontrollable' and localized discharge to the cathode. In combination with the other elements of my tube, this assists in reducing cathode disintegration to a minimum and allows long life, considered otherwise improbable with a vapor tube operating at a potential above the critical potential of the ionizable medium. With the grid coating and structure used, this cadmium amalgam tube allows a high voltage amplification factor and low impedance, resulting in a. tube of ex- The vapor pressure is adequate to re-' ceptionally high mutual conductance and power sensitivity.

As in th tubes of my two aforesaid co-pending applications, the\ discharge should be confined to the space between the cathode emitting surface and the anode and diretly through the This is accomplished byproviding blockmembers, such as mica discs at the anode ends of the cathode emitting surface terminals, transverse electrical field of such terminals as provided will flow to length is less than the diffused uncontrollable discharge free path supported by lead terminal (2b). Closely surrounding the cathode is control grid (3) supported by lead (3b), and support (30) and which has an inside diameter about mils greater than that of the outside diameter of the cathode. It has a winding of twenty-eight turns per inch along the cathode axis and extends for the length of the cathode emission surface. Surrounding this at a distance of approximately 50 mils is shield grid (4) which has the same number of turns as grid (3) but of a wider diameter. Grid (4) is supported by lead (4b) and support (40). Surrounding grid (4) and spaced about 50 mils therefrom, is grid (5) of similar construction. but wider diameter. Grid (5) is supported by lead (5b). The nickel anode (6) is of .750" diameter and at its terminals appear mica (9) for confining the discharge to tween the cathode and anode maintain the elements are held in The cathode is heated ('1) which has an integrally formed coating of aluminum oxide to insulate it from The capsule (10) contains cadmium-mercury amalgam and a getter such as magnesium. (RF) is the radio frequency which is applied to the grid (3) and cathode (2) through the negative biasing portion of resistance (R1) which is shunted across plate potential supply (E allowing a positive potential to be applied to shield grids At (C) is the output condenser for the anode potential supply and at (S) a translating device, such as a loud speaker.

While the cathode shown is indirectly heated, a directly heated type may, of course, be used.

The assembled tube should be completely degasified and the alkaline earth oxides broken down to their active form as is general practice, the magnesium getter and the mercury amalgam pill being discharged into the tube.

The third grid potential can be varied within limits so as to be positive or negative with respect to the second grid. g

In operation, the cathode (2) is raised to emission temperature, and electrons are discharged therefrom. The control grid (3), being located within the dense electron field immediately adjathe space beand which help to.

is negatively low value.

fier tubes.

While most of the above description of the tube relates to its employment as a detector, it may, however, be used as a tube having a very high power sensitivity.

The tube electrons in 2. An ionic discharge amplifier claim 1 in claim 1 in in claim 7 in which the ionizing medi wire woun 6. An ionic discharge amplifier claim 1 in which the anode surrounds the cathode claim 1 in which the control elements are of the d type.

rid

8. An ionic discharge amplifier as described in claim 7 in which the control grid is adapted to be negatively biased in respect to the cathode.

and control grid.

10. An ionic discharge amplifier as described in claim 7 in which the grids are of the wire wound pe 

